Several clonal strains of rat pituitary tumor cells (GH-cells) synthesize and secrete into the medium different amounts of two hormones, prolactin (PRL) and growth hormone (GH). The synthesis of these two proteins can be further modulated by externally added physiological and pharmacological agents. Understanding of the mechanism/s involved in the graded basal level production of the hormone prolactin (PRL) and the regulation of its synthesis by thyrotropin releasing hormone (TRH), hydrocortisone (HC), estradiol (E2) and the drug 5-bromodeoxyuridine (BrdUrd) in GH-cells is the primary objective of this proposal. Though the model cell system utilized in this investigation is not composed of normal cells, it still responds to physiological agents such as TRH, HC and E2 in the same manner as does the normal pituitary gland. Since distinct cell types are believed to produce specific hormones in the normal pituitary gland, the GH-subclones which produce two hormones (PRL and GH) provide a novel model system for the understanding of mechanisms whereby neoplastic cells can express a portion of the genetic information that is kept under strict control in normal cells. The synthesis of PRL by a distinct type of cells in the normal pituitary gland represents a differentiated function. The different GH-subclones which produce vastly different amounts of PRL, provide an ideal system for the investigation of the cellular mechanisms which lead to the regulated synthesis of such a cell specific protein and thus provide novel means to understand the process of cellular differentiation. The striking effect of BrdUrd on PRL synthesis will be utilized to obtain useful information on the nature of hormone receptor interactions with specific DNA binding sites. Cell-free systems in which events resembling specific RNA and protein synthesis occur, are established in this Laboratory. Such cell-free systems will be utilized to locate the control points of regulated synthesis of a hormone in pituitary tumor cells.